Shape of the lateral edge of the first metatarsal head changes depending on the rotation and inclination of the first metatarsal: a study using digitally reconstructed radiographs

Background: Correction of pronation deformity and metatarsal primus varus is an important component of hallux valgus surgery, necessary to achieve a satisfactory correction and to prevent post-operative recurrence. Roundness of the lateral edge of the first metatarsal head (round sign) on the dorsoplantar radiograph of the foot has been empirically advocated as an indicator of first metatarsal pronation. The purpose of this study was to clarify the effect of rotation and inclination of the first metatarsal on the shape of the lateral edge of the first metatarsal head. Methods: Computed tomographic images of feet in 30 subjects, without pathology of the first metatarsophalangeal joint, were included. Digitally reconstructed radiographs of the first metatarsal were created using the computed tomographic data. Thirty-nine images were created of each first metatarsal at different degrees of rotation (−10° to 30° of pronation) and inclination (−10° to 20° of plantarflexion). Then, the shape of the lateral edge of the first metatarsal head was classified into three types: angular, intermediate, and round. Generalized estimation equations were used to test if the shapes of the first metatarsal head were significantly different across the range of pronation and plantarflexion angles. Results: The positive round sign changed to negative as the first metatarsal supinated. In most feet, these changes occurred as the pronation angle decreased from 10° to 0°. The positive round sign also changed to negative as the first metatarsal head plantarflexed. Conclusion: Positive round sign of the first metatarsal head on the dorsoplantar radiograph of the foot was significantly associated with increased pronation as well as decreased inclination of the first metatarsal. Negative round sign may be used as an indicator of effective correction of first metatarsal pronation during hallux valgus surgery.